1. Field of the Invention
The present invention relates to a lens control device adapted for use in an image pickup device such as a video camera or an electronic still camera.
2. Related Background Art
In the image pickup devices such as a video camera, there has conventionally been employed a focusing method of detecting the sharpness of the image frame from an image signal released from said image pickup means and controlling the position of a focusing lens so as to maximize said sharpness.
The sharpness is generally evaluated by the intensity of high frequency components extracted by a band-pass filter (BPF) from the image signal or the detected intensity of unfocused width of the object image, detected, for example, by a differentiating circuit from the image signal.
In case of taking an ordinary object, during an unfocused state with a low sharpness, the high frequency components are weak and the unfocused width of the object image is large. As the lens approaches to the focused state, the high frequency components assume a higher level and the unfocused width becomes smaller, and, in the completely in-focus state, the high frequency components assume a highest value while the detected unfocused width becomes smallest, whereby the sharpness becomes maximum.
Consequently, a focusing operation can be achieved by moving the focusing lens as fast as possible, while the sharpness is still low, in such a direction as to increase the sharpness and decelerating the focusing lens according to the increase of sharpness so as to stop the system on top of the peak of sharpness, whereby the focusing lens can be precisely stopped at the in-focus position. The automatic focusing method with such focusing lens control is generally called peak climbing automatic focusing (AF) method.
Also in association with the recent development of video cameras or the like with more complex functions, there is proposed so-called wide range automatic focusing system with a widened focusing range. In this system, the movable range of a conventional optical system is widened toward the shorter range side (macro region), thereby enabling focusing to an object at a shorter distance.
However, in the wide range automatic focusing system utilizing the above-mentioned peak climbing automatic focusing method, the focusing operation may not be conducted properly immediately after the start of power supply, if the focusing lens is positioned at the shortest object range side when the power supply is previously turned off. For example, there may be encountered drawbacks that the focusing lens is stopped at an out-of-focus position or that the focusing operation requires a long time. The object to be focused immediately after the start of power supply is empirically most often positioned in the ordinary distance range, while the focusing lens in the shorter object distance region requires a larger amount of movement, for a given change in the object distance, in comparison with the focusing operation in the ordinary object distance region. For these reasons, the system is often unable to follow the object in the ordinary object distance region while the focusing lens is in the shorter object distance region, thus eventually requiring a long time for escaping from such state or erroneously stopping the focusing lens before the focusing operation is completed.